There was a sound reason why the opening of the Darwin Centre at the Natural History Museum last month was based around butterflies. In 1848, when Darwin was writing The Origin of Species, two young fans admired his account of The Voyage of the Beagle to the point of imitation, and set off to discover what could be seen up the Amazon. Henry Bates and his friend Alfred Russel Wallace first met in Leicester public library, poor autodidacts whose education had been broadened by the new mechanics' institutes. They were enthused by Brazilian biodiversity, demonstrated by 700 butterfly species within a lethargic toddle of base camp, and surprised that, despite such multiplicity, members of the cabbage white family Leptalis should mimic the brash colours and patterns of the longwing Heliconidae. Bates immediately twigged that this was Darwin's theme of "adaptation". The Heliconidae feared no predator; their wingscale artworks advertised that they were a species that smelt sharp and tasted nasty.The Leptalis, odourless and palatable, flapped around displaying the false colours of Heliconidae – and thereby lived long enough to pass on their genes.

Not that Darwin, Bates or Wallace then understood that genetic mechanism of selection or the biochemistry of change. But they were thrilled enough by mutability in itself, which enabled a butterfly to impersonate a dead leaf, or a brood of Papilio swallowtails (as observed by Wallace later, in Indonesia) to burst out of the chrysalis marked to replicate the many local species repugnant to predators.

Mimic butterflies joined fossils and variant island finches in demonstrating to Darwin and his collaborator Wallace, co-definer of natural selection, that change was the norm of life on Earth. Not onwards and upwards, either, just ever onwards, reactive and improvisatory, perfection being merely a brief moment in a continuum.

Most of, and the best of, Forbes's book is given over to the scientists who thereafter worked out the operation of heredity, chromosomes, genes, DNA, and the chemical determinants that activate those processes. A century and a half later, the precise mechanisms by which Leptalis impersonate Heliconidae, every pixel of a wingscale tinted convincingly right, are still not certain: the jury is also out on whether Leptalis changed dramatically, or incrementally. The wisest scientists, all of whom it's a pleasure to meet on the page however swiftly they flit past, asked questions of brilliant novelty. Miriam Rothschild needed to know from which plants the model butterflies got their toxins, and just how noxious those substances were (she extracted the poisons and fed them to birds; even toughie starlings vomited): then she worked out that the butterfly eggs and caterpillars had evolved their own imperviousness to the toxic plant – they alone could eat it, and during their exclusive browse absorbed the protective toxicity.

Forbes isn't content with the awesomeness of mimic butterflies, though, or with the real truth about research into the modifications of the pigmentation of peppered moths after the Clean Air Act. He also quickly covers such amazements as the camouflage potential of the octopus: the colourblind octopus assesses its environment visually, then its brain directly transmits to the shutters of the black, red and yellow colour cells of its skin exactly which combination of open and shut will blend its soft succulence into a background, helped by a deep dermal layer that reflects back whatever colour hits it. Like a superior interior designer, the octopus doesn't just do colour, it does texture as well, smoothing or spiking skin projections to match. And it can replicate stippled, mottled or disruptive patterns, and even the scary big "eye" images some butterflies have evolved. No creator of camouflage for warfare could hope to hide or disguise hardware or troops to that level of discretion.

Forbes does tackle camouflage in 20th-century warfare, but again he's interested in its biological connections and its scientists, although in fact the first to connect animal invisibility and planned reticence on the human battlefield was a dotty New England artist, Abbott Thayer. His observations were the basis of Thayer's law of concealing coloration, establishing that creatures tend to be dark on their backs, paling towards the belly, so as to flatten light and shade on a three-dimensional object. Thayer and the Scots zoologist John Graham Kerr proselytised before and during the first world war for the lies that paint could tell about men, materiel and ships, and Hugh Cott, Kerr's direct Glaswegian successor, wrote the subject's textbook, Adaptive Coloration in Animals, published in 1940.

Forbes is especially shrewd about the institutional infighting that made camouflage suspect with the military. However, Cott's natural world principles, categorised as merging, disruption, disguise, misdirection, dazzle, decoy, smoke screen, dummies and false displays of strength, did become the basis for subsequent military camouflage, starting with successes improvised in the North African desert campaigns with palm fronds and jerry cans. Those improvisations knocked up by a rum mix of biologists and artists fascinate Forbes, because he sees with lovely clarity that nature, like art, is a bricoleur, a tinkerer, and that the thrill of it all is not in a stately grand design – as Darwin understood, there never has been any such thing, it's all expendable – but in life's multiple choices, chances and smallscale experiments: so many possibilities.